Welcome To Camp Nightmare (Goosebumps, #9) by R.L. Stine

This is a good one! Starts out totally creepy and doesn't let up until the end. Billy narrates his own story of summer camp and weird things happen right from the bus trip out there, things get dangerous, spooky, kids disappear, etc. until the final twist ending. Which is pretty good even though I did figure it out but what you don't see coming is the extra, extra twist on the last page, last paragraph that cues that awesome Twilight Zone music for a super horror story ending. Fun!